1. Field of the Invention
The present invention generally relates to an apparatus and method that enables one or more modems, connected at a premises to a single subscriber line, to simultaneously communicate data with a modem at the central office end of the subscriber line, using non-symmetrical half-duplex data transmissions. Non-symmetrical half-duplex data transmissions include half-duplex transmission on a subscriber line wherein the transmission time in one direction is different than the other direction and/or the transmission data rate in one direction is different than the other direction.
2. Background of Related Art
Data communication on a subscriber line is typically referred to as DSL (digital subscriber line) communication. Examples of DSL technologies are ADSL, RADSL, HDSL, VDSL, basic rate ISDN, etc. Currently, most DSL communication is full duplex. Full duplex DSL communication is usually achieved on a wire pair by either frequency division multiplex (FDM), echo canceling duplex (ECD), or time division duplexing (TDD).
In FDM, the physical layer transmissions in each direction of communication utilize separate frequency bands with a guard band between these two communication bands. A result is that FDM requires more than twice the channel bandwidth than that required for just one communication direction. An additional consequence of FDM is that FDM suffers increased channel loss and reduced performance in one direction.
In ECD, the physical layer transmissions in both directions of communication utilize echo canceling in the same frequency band used to separate transmit and receive signals. A result is that ECD is susceptible to non-linear distortion and other non-cancelable aspects of the transmitted signal with a consequence that ECD suffers decreased dynamic range and reduced performance in both directions of communication.
In TDD, the physical layer transmissions alternate in one direction, then the other direction, in pre-arranged, equal time periods. In TDD, both directions of communication utilize the same frequency band and do not require echo canceling, thus avoiding the above disadvantages of FDM and ECD. However, TDD suffers the disadvantage of the maximum data rate in each direction of transmission is one-half that achievable in only one direction. In FDM, ECD and TDD, the physical layer transmissions are decoupled from and independent of the higher communication layers.
A full duplex variation restricted to communicating Ethernet data and called “EtherLoop” (developed by Elastic Networks) uses FDM, but couples transmissions to only Ethernet messages. Generally, DSL communication in the prior art is point-to-point, in that there is a single operating DSL modem at each end of the subscriber line with no provision for multiple DSL modems to be able to operate at either end.
Some leased line voiceband modems in the prior art provide for a single central site modem which communicates with one or more remote modems: a concept referred to as “four-wire multipoint communications.” An example of such a modem is one that complies with the industry standard ITU V.27bis. The communication channel to each remote modem is a four-wire connection and the modems are typically widely geographically dispersed over the public telephone network. It is important to note in the dial-line modem prior art, the central site modem transmission is controlled by an attached central site computer or data terminal, which uses non-data control signals. An example of non-data control signals are those prescribed in industry standard ITU V.24 CT105 to control the start and end of transmissions.
Similarly, some dial line voiceband modems in the prior art provide for a single central site modem which communicates with one or multiple remote modems, which is a concept referred to as “two-wire PSTN communications.” The communication channel to each remote modem is a two-wire PSTN connection and the modems are typically, widely geographically dispersed over the public telephone network. The physical layer is half-duplex, and the data protocol is half duplex. The direction of transmission is determined external from the dial modem and modem transmissions are thus controlled externally by control signals, such as Request-to-Send or V.24 CT105.
Importantly, in both prior art voiceband modem cases discussed above, the central site modem and the remote modems are never at both ends of a single subscriber loop.
Another example is for remote transmission controlled by an attached remote computer or data terminal which also uses non-data control signals such as those described in industry standard ITU V.24 CT105 to control the start and end of transmissions. In the leased line prior art cases, the central site transmission is continuous and the remote transmission is controlled as in the dial line case. In both these cases, the attached computers or data terminals must ensure that transmissions do not overlap by monitoring the ends of received signals, for example via ITU V.24 CT109, and delaying CT105 accordingly.
It should be noted that, with respect to the prior art voiceband modem discussed above, communications for one or more remote users are not on the subscriber line and involve transmission controls from attached multiple computers or data terminals via non-data interfaces. It may be constructive to note that the dial modem techniques are not efficient for use on a subscriber line where much higher data rates and faster turnaround times are demanded.
Another prior art technology is Ethernet local area network communication wherein the physical channel can be a short two-wire channel, never a general subscriber line. Here, transmissions are derived directly from the upper layer data protocol, but there is no central control, and therefore, the signals may collide. A special upper layer protocol must manage the detrimental effects of collisions. It may also be constructive to note that the Ethernet techniques cannot be efficiently applied to subscriber loops because of collisions and the ability to span the distances incurred on a loop.
Another aspect in the prior art is that the Point-to-Point Protocol (PPP) defined in Internet Engineering Task Force (IETF) specification Request for Comment (RFC) 1661 is defined to operate as follows: “The Point-to-Point Protocol is designed for simple links which transport packets between two peers. These links provide full-duplex simultaneous bi-directional operation.” Up to this time, the Point-to-Point Protocol has not been used in connection with half-duplex links.
Heretofore, modems have lacked the ability to conduct half-duplex transmission on a subscriber line wherein the transmission time in one direction is different than the other direction and/or the transmission data rate in one direction is different than the other direction.